1. Field of the Invention
The present invention relates to a pressure sensor and, particularly, to a shielding structure for preventing a pressure leak in a pressure introduction portion.
2. Description of the Prior Art
FIG. 7 is a partial sectional view of a conventional pressure sensor 20 disclosed by Japanese Laid-open Patent Application No. 5-312661, for example. This conventional pressure sensor 20 has a shielding structure used for the measurement of the pressure of fuel, mainly gasoline, or fuel vapor for an internal combustion engine. In FIG. 7, reference numeral 1 denotes a pressure sensitive element for detecting pressure by the piezoresistance effect of a resistor on a thin film silicon chip formed like a bridge. Reference numeral 2 represents a substrate for mounting the pressure sensitive element 1 and electric parts such as an unshown amplifier circuit and compensator circuit. Reference numerals 3 and 4 indicate O rings for shielding pressure P in a lower part of the figure so that it does not leak into portions therearound when the pressure P is transmitted to the pressure sensitive element 1 through a nipple 5. The O ring 3 made from a fluorine-based material having excellent gasoline resistance is used on the side of a medium to be detected for its pressure whereas the O ring 4 made from a silicon-based or, phlorosilicon-based material is used to ensure shielding properties at low temperatures at which the shielding properties of a fluorine-based material is inferior. Denoted by 6 is a filter for removing dust contained in the pressure P, 7 a base and 8 a cover. Reference numeral 9 denotes a connector, projecting from the base 7, for receiving power from an unshown computer, supplying it to the pressure sensitive element 1, the amplifier circuit and the compensator circuit, and applying an output signal in accordance with the pressure P to the computer from the pressure sensitive element 1 through the amplifier circuit and the compensator circuit. Reference numeral 10 indicates a cup connected to a recessed portion 11 located right below the pressure sensitive element 1 and projecting from the base 7 at a peripheral portion in contact with the base 7 to prevent the breakage of a shield after it is installed in the recessed portion 11 with the filter 6 interposed therebetween and having an O ring storage chamber 12 which is open at the top and a through hole 13 connected to a bottom portion of the O ring storage chamber 12. Reference numeral 14 denotes a pipe welded to guide the pressure P to the pressure sensitive element 1, and 15 a holder located on the substrate 2 side of the holder 14 to prevent the projection of the O rings 3 and 4.
A case where a shielding structure consisting of the O rings 3 and 4 is set between the cup 10 and the pipe 14, both forming a pressure introduction portion, will be described. When the cover 8 is not placed upon the base 7, the O rings 3 and 4 are fitted onto the pipe 14, which projects downward from the holder 15 below the substrate 2 mounting the pressure sensitive element 1, the amplifier circuit and the compensator circuit, in parallel to each other in an axial direction. The pipe 14 is then inserted into the through hole 13 from the 0 ring storage chamber 12 of the cup 10 installed on the base 7, whereby the O rings 3 and 4 are inserted in such a manner that they are pressed by the holder 15 and elastically deformed between the outer peripheral surface of the pipe 14 and the inner peripheral surface of the O ring storage chamber 12. Thereafter, the substrate 2 mounting the pressure sensitive element 1 and electric parts and parts such as the O rings 3 and 4, the filter 6, the cup 10, the pipe 14 and the holder 15 are stored in internal space 16 formed hermetically by combining the base 7 and the cover 8 together.
A description is subsequently given of the operation of the pressure sensor. The pressure p is transmitted from the nipple 5 to the pressure sensitive element 1 through the filter 6 where dust is removed and the pipe 14. At this point, the pressure P is precisely transmitted to the pressure sensitive element 1 by the action of the O rings 3 and 4 without leaking into the internal space 16 from the gap between the cup 10 and the pipe 14. While the pressure sensitive element 1, the amplifier circuit and the compensator circuit receive operation power from the computer through the connector 9, an output signal calculated by the amplifier circuit and the compensator circuit in accordance with the pressure P detected by the pressure sensitive element 1 is applied to the computer from the connector 9.
As the shielding structure of the conventional pressure sensor 20 is constructed as described above, the hardness of the O rings may be restricted by the characteristic properties of the materials of the O rings from the relationship between them and the medium to be detected for its pressure. For example, if the O ring 3 has a hardness of 20 (JIS Hs) or more, as shown in FIG. 8, there is a case where the O ring 4 receives stress from the O ring 3 and the holder 15 and projects from the gap between the O ring storage chamber 12 and the holder 15 when the pipe 10 fitted with the O rings 3 and 4 is inserted into the cup 10. If the O ring 4 projects, a projecting portion 17 of the O ring 4 is inserted between the cup 10 and the holder 15.